Influence of Graphene Nano Particles and Antioxidants with Waste

energies

Article Inﬂuence of Graphene Nano Particles and Antioxidants with Waste Cooking Oil Biodiesel and Diesel Blends on Engine Performance and Emissions

Sandeep Krishnakumar 1 , T. M. Yunus Khan 2,3,* , C. R. Rajashekhar 1, Manzoore Elahi M. Soudagar 4 , Asif Afzal 5 and Ashraf Elfasakhany 6

1 Mechanical Engineering Department, Mangalore Institute of Technology and Engineering (MITE), Moodabidre 574225, Karnataka, India; [email protected] (S.K.); [email protected] (C.R.R.) 2 Research Center for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box 9004, Abha 61413, Asir, Saudi Arabia 3 Department of Mechanical Engineering, College of Engineering, King Khalid University, Abha 61421, Saudi Arabia 4 Department of Mechanical Engineering, Glocal University, Mirzapur Pole 24712, Saharanpur District, Uttar Pradesh, India; [email protected] 5 Department of Mechanical Engineering, P. A. College of Engineering, Visvesvaraya Technological University, Mangaluru 574153, India; [email protected] 6 Mechanical Engineering Department, College of Engineering, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia; [email protected] * Correspondence: [email protected]

Citation: Krishnakumar, S.; Khan, Abstract: The main reason for the limited usage of biodiesel is it tends to oxidize when exposed to T.M.Y.; Rajashekhar, C.R.; M. air. It is anticipated that the addition of an antioxidant along with graphene nano particle improves Soudagar, M.E.; Afzal, A.; combustion of diesel-biodiesel blend. In the present research biodiesel made from the transesterifi- Elfasakhany, A. Influence of cation of waste cooking oil is used. Three synthetic antioxidants butylated hydroxytoluene (BHT), Graphene Nano Particles and 2(3)-t-butyl-4-hydroxyanisole (BHA) and tert butylhydroquinone (TBHQ) along with 30 ppm of Antioxidants with Waste Cooking Oil Biodiesel and Diesel Blends on graphene nano particle were added at a volume fraction of 1000 ppm to diesel–biodiesel blends Engine Performance and Emissions. (B20). The performance and emission tests were performed at constant engine speed of 1500 rpm. Energies 2021, 14, 4306. https:// Because of the inclusion of graphene nano particles, surface area to the volume ratio of the fuel is doi.org/10.3390/en14144306 augmented enhancing the mixing ability and chemical responsiveness of the fuel during burning causing superior performance, combustion and emission aspects of compression ignition engine. Academic Editor: The results revealed that there was a slight increase in brake power and brake thermal efficiency George Kosmadakis of about 0.29%, 0.585%, 0.58% and 6.22%, 3.11%, 3.31% for B20GrBHT1000, B20GrBHA1000 and B20GrTBHQ1000, respectively, compared to B20. Additionally, BSFC, HC and NOx emissions were Received: 22 May 2021 reduced to considerable levels for the reformed fuel. Accepted: 14 July 2021 Published: 16 July 2021 Keywords: biodiesel; nano material; antioxidants; oxides of nitrogen (NOx)

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional afﬁl- 1. Introduction iations. The demand as well as cost of the petroleum fuel is increasing day by day. One of the foremost possible alternate fuel for compression ignition engine is biodiesel. Narrow reserves of petroleum-based fuel and environmental concern have set off researchers to ﬁnd substitute fuel that replaces the petroleum-based fuel. Diesel consumption was at Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. 81.1 million tons in 2017–2018 and 76 million tons in 2016–2017. Total auto fuel consumption This article is an open access article during 2018–2019 was 4% higher, diesel consumption was 3% higher than 2017–2018 [1]. distributed under the terms and Heavy vehicles still depended on diesel as a main source of power. The main hindrance conditions of the Creative Commons with diesel fuel at present is its exhaust tail pipe emission and the fear of depletion of Attribution (CC BY) license (https:// petroleum fuel. creativecommons.org/licenses/by/ In this situation, energy from biofuels delivers a bright spot to compensate for the 4.0/). imports serving a good substitute for the fossil fuels [2]. Biodiesel is renewable, oxygen

Energies 2021, 14, 4306. https://doi.org/10.3390/en14144306 https://www.mdpi.com/journal/energies Energies 2021, 14, 4306 2 of 17

rich, nontoxic, sulphur free clean burning fuel and contains no aromatic compounds. Conceivably there are quite a few drawbacks associated with biodiesel as a vehicular fuel, the major one is its inability to resist oxidation [3], since it degrades more when exposed to atmospheric oxygen and forms hydroperoxides which produce insoluble gums and sediment gums that chokes the injector and after oxidation viscosity of the fuel increases and results in poor atomization. Based on research conducted earlier, it is learnt that the biodiesel blend as a fuel along with diesel, minimizes emissions such as carbon monoxide, hydrocarbon, sulphur oxides and particulate matter but there will be a rise in oxides of nitrogen [4]. The present research lies in addressing the oxides of nitrogen emission without compromising the power and efficiency of biodiesel blended diesel fuel. Oxides of nitrogen are one of the main forerunners of acid rain, which has serious physical and environmental effects [5]. The majority of the nitric oxide (NO) emission is anthropogenic. However, the major source of oxides of nitrogen emissions are through vehicles. According to the Zeldovich equation [6], nitric oxide is the main preparator in the formation of oxides of nitrogen during combustion. The two main factors that contribute to the formation of oxides of nitrogen is oxygen content in the fuel and combustion temperature above 760 ◦C. N2 + O → NO + N N + O2 → NO + O N + OH → NO + H It is known that: oxides of nitrogen formation happen in three prospects during combustion, i.e., thermal NOx, fuel NOx and prompt NOx. Thermal NOx is formed due to the amalgamation of nitrogen and oxygen that forms the main portion of the oxides of nitrogen development. Intensity of thermal NOx formation rate becomes sharp with the increase in temperature above 1300 ◦C[7]. At this temperature the ‘O’ atom of the oxygen breaks the strong triple bond of the nitrogen, followed by O2 and OH oxidizes the N atom [8], which leads to the formation of oxides of nitrogen. Fuel NOx formed because of the contamination of nitrogen in the fuel, which contributes about 50% of oxides of nitrogen produced [7]. Nitrogen in the fuel undergoes thermal decomposition by O, H and OH leading to NHi. NHi molecules then undergo either oxidation to nitric oxide or reduction with NO to molecular nitrogen [9]. Prompt NOx is the oxidation of the nitrogen in the air with the fuel, in a fuel rich condition. An important aspect of both prompt NOx and fuel NOx formation is the conversion of HCN (hydrogen cyanide) to NO [10]. CH + N2 ↔ HCN + N HCN + O ↔ NCO + H NCO + H ↔ NH + CO NH + H ↔ N + H2 N + OH ↔ NO + H ◦ Thermal NOx is absolutely very low below 1300 C, and rises sharply with temperature. Fuel NOx does not vary with temperature. Prompt NOx increases marginally with temperature. Furthermore, the oxides of nitrogen formation also highly depend on the type of fuels, its physiochemical properties, injection parameters, inlet temperature and pressure and compression ratio of engine [11]. One of the methods for improved combustion and minimize perilous emission is through the fuel reformulation technique using fuel additive. Inclusion of nano particle possess large surface area to volume ratio of the fuel, which increases the catalytic reactivity thus improves combustion and curtail emissions. Copious research has been done on the effect of metal-based additives, carbon-based additives, metallic oxides, oxygenated additives, etc. Graphene nanoparticles in the dosage of 50, 75 and 100 ppm on transesterified palm oil showed higher thermal efficiency of about 2.5% and reduction of unburnt hydrocarbon, carbon monoxide and a small rise in the emission of oxides of nitrogen of about 3.8% was observed. The presence of graphene reduced the ignition delay and improved the premixed combustion [4]. Energies 2021, 14, 4306 3 of 17

Graphene oxide added dairy scum oil methyl ester (DSOME) showed improved performance in terms of rise in thermal efficiency, drop in brake specific fuel consumption and significant reduction in unburnt hydrocarbon, carbon monoxide and smoke. Addi- tionally, considerable decrement in duration of combustion and ignition delay period was observe [12]. By adding graphene nanoparticle in a suitable proportion to simarouba biodiesel blend could improve overall performance and emission of ci engine without engine alteration [13]. The use of GO nano particle to Oenothera lamarckiana biodiesel blend shows considerable reduction in carbon monoxide (5–22%), unburnt hydrocarbon (17–26%) and slight increase in carbon dioxide (7–11%) and oxides of nitrogen (4–9%) were observed in ci engine [14]. The result of alumina nanoparticle to mahua biodiesel bare subsequent reduction in the exhaust emissions such as carbon monoxide, hydrocarbon as well as smoke and slight improvement in the thermal efficiency was observed [6]. Better mixing and rate of reaction was found with the addition of alumina and cerium oxide nano particle of 30 ppm each with diesel-biodiesel blends [15]. Different monophyletic antioxidants like p-phenylenediamine, ethylenediamine, BHT and L-ascorbic acid showed substantial reduction in oxides of nitrogen for biodiesel [16]. Copious research has been done in improvising the oxidation stability of biodiesel and also to reduce the detrimental emission of the biodiesel diesel blend. The effect of addition of 2,6-di-tert-butyl-4-methylphenol and 2,20-methylenebis antioxidants of 1% by volume to Calophyllum inophyllum based biodiesel showed good stability to resist the oxidation of the fuel. The outcome also showed the effectiveness of antioxidants in enhancing the start of combustion by shortening the ignition delay and improvement in power, efficiency and reduced specific fuel consumption. Additionally, both antioxidants considerably reduced oxides of nitrogen emission. Conversely, CO, HC and smoke opacity were slightly increased [17]. Two monophenolic antioxidants butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA) at 1000 ppm and 1500 ppm concentration to B20 biodiesel blend derived from palm oil showed mean increase in brake specific fuel consumption, reduced brake thermal efficiency and reduced nitrogen oxide emission compared to B20 [3,18]. Addition of N-N’-diphenyl-1,4-phenylenediamine (DPPD) antioxidant to jatropha biodiesel reduced oxides of nitrogen, carbon monoxide, hydrocarbon emission with a trivial rise in terms of brake power and BSFC [19]. Profuse research findings have been done in observing the effect of carbon-based additive as well as antioxidants separately in overcoming the main limitation of biodiesel usage in engine. However, no report findings are available that elucidates both. In the present research, the influence of both graphene and antioxidants together is presented on performance and emission of diesel–biodiesel blend. From the previous literature it was observed that the addition of carbon-based nano particle as a fuel borne catalyst (graphene and carbon nano tube) could improve engine performance parameter and emission attributes in comparison with the pure diesel and biodiesel blend (B20). However, the majority of the researchers reported slight augmentation in oxides of nitrogen emission that needs to be addressed. On the other hand, from the previous studies it was also reported that use of antioxidants not only controls oxides of nitrogen emission but also causes poorer power development as well as a surge in specific fuel consumption. The innovative future of the present study aimed at experimental exploration with the unified effect of graphene nano particle with antioxidants in controlling the exhaust emission without compromising performance of the engine. For the present work, commercially available research grade graphene was purchased from Adnano Technology. Three monophenolic synthetic antioxidants butylated hydroxytoluene (BHT), 2(3)-t-butyl-4-hydroxyanisole (BHA) and tert butylhydroquinone (TBHQ) were purchased from Sigma-Aldrich. Energies 2021, 14, 4306 4 of 17

2. Materials and Methods 2.1. Graphene Nano Material Graphene is ultra-light weighed with less defected honey comb 2D carbon structuring and having minimum number stacking with high graphitized carbon. Technical speci- ﬁcations of graphene are given below in Table1. Exactly 30 ppm of the graphene was considered for test fuel preparation.

Table 1. Technical speciﬁcation of the graphene.

Purity 99% Thickness (Z) 0.8–1.6 nm Dimension (X and Y) <1 µm Number of Layers 1–5 Surface Area >200 m2/g Bulk Density 0.006 g/cm3

2.2. Antioxidants Antioxidants hinder the oxidation of the unsaturated fatty acids in the biodiesel and renders the formation of redundant yield. There are three basic types of antioxidants: phenolic, amines and thiophenols. Phenolic types of antioxidants are extensively used in the biodiesel industry because of their availability, low cost and feasibility.

2.3. Test Fuel Preparation Biodiesel considered for the present study was derived from the transesteriﬁcation of used cooking oil, which was collected from the Center for Renewable Energy and Sustain- able Technology (CREST) at the National Institute of Engineering, Mysuru. The catalyst and alcohol used for the transesteriﬁcation were potassium hydroxide and methanol, respectively. The waste cooking oil was heated to 65 ◦C after mixing with alcohol and catalyst for about 3 h with constant stirring. The formed glycerine was then removed and the pH value of 7 was maintained for methyl ester (formed biodiesel). Biodiesel of 20% by volume (B20) was blended with diesel with the aid of magnetic stirrer for about 20 min at 1000 rpm. Exactly 30 ppm of graphene nano particle was weighed by precision electronic balance. Sodium dodecyl sulphate acts as an anionic surfactant for graphene nano particle. The role of anionic surfactant was considered to reduce the agglomeration tendency of graphene particle [20]. Graphene and surfactant were ultrasonicated for about 20 min for the equal dispersion of graphene in the fuel. Three antioxidants BHT, BHA and TBHQ of 1000 ppm concentration were considered separately with 30 ppm of graphene to prepare different test fuel samples. Each antioxidant was weighed and dissolved in ethanol. Then, they were added separately with the graphene nano particle. These mixtures were incorporated to the biodiesel blend and the improved dispersion was accomplished with ultrasonication followed by magnetic stirring.

2.4. Fuel Physical Property Fuel physical properties of biodiesel were measured and compared with ASTM biodiesel standard as shown in the Table2. Energies 2021, 14, 4306 5 of 17

Table 2. Fuel physical property in comparison with standard.

Biodiesel Waste ASTM BBD + GR- BBD + GR- BBD + GR- from Waste Cooking Oil Properties Unit D7467 Diesel BHT1000 BHA1000 TBHQ1000 Cooking Oil Biodiesel Standard ppm ppm ppm (B20) (B100) Flash point ◦C Min 52 65 71 160 79 79 79 Kinematic 1.9–4.1 CST 2 to 3.8 4.06 4.2 4.05 4.05 4.05 Viscosity, 40 ◦C (ASTM D93) Caloriﬁc Value kJ/kg N/s 42,600 41,320 36,848 40,750 40,090 40,090 Density kg/m3 N/s 820 880 920 880 880 880

3. Engine Speciﬁcation and Test Procedure The experimental investigation was carried out in the engine lab, department of mechanical engineering at VVCE, Mysuru, India. Experimentation was performed on a research engine. Engine speciﬁcation is shown in Table3.

Table 3. Engine speciﬁcation.

Product 1 Cylinder, 4 Strokes, Multifuel, VCR Research Engine Engine Kirloskar made Type of cooling water cooled Stroke, Bore, Cubic capacity 110 mm, 87.5 mm, 661 cc Rated Power 3.5 kW at 1500 rpm Dynamometer Type eddy current, water cooled with loading unit Load sensor Make VPG Sensotronics, Load cell, type strain gauge, range 0–50 Kg Overall dimensions W 2000 × D 2500 × H 1500 mm

Exhaust emission were measured in AVL DIGAS 444N, which was integrated to the engine. The experimental investigation involved a comparison study of combustion, performance and emission characteristics of diesel, biodiesel blend (B20) and graphene plus antioxidant added biodiesel blend. Blending of biodiesel was kept at 20% constant with diesel. Fuel blends are shown in Table4.

Table 4. Fuel blends description.

B20 20% Biodiesel from Waste Cooking Oil + 80% Diesel 20% Biodiesel from waste cooking oil + 80% Diesel + 30 PPM of Graphene + 1000 ppm of B20GrBHT1000 BHT antioxidant 20% Biodiesel from waste cooking oil + 80% Diesel + 30 PPM of Graphene + 1000 ppm of B20GrBHA1000 BHA antioxidant 20% Biodiesel from waste cooking oil + 80% Diesel + 30 PPM of Graphene + 1000 ppm of B20GrTBHQ1000 TBHQ antioxidant

The layout of the research engine with dynamometer and exhaust gas analyser is shown in Figure1a. Photographic view of the experimental set up is shown in the Figure1b. Energies 2021, 14, 4306 6 of 17 Energies 2021, 14, 4306 6 of 17

(a) (b)

FigureFigure 1. 1. (a(a) )Layout Layout of of experimental experimental setup; setup; (b (b) )Photographic Photographic view view of of experimental experimental setup. setup.

4.4. Uncertainty Uncertainty Analysis Analysis of of the the Experimental Experimental Data Data TheThe accuracy accuracy of of the the engine engine parameter parameter and and un uncertaintiescertainties in in the the collected collected data data from from the the testtest cell cell of of the the engine engine is is given given in in Table Table 55.. The error size was reduced by considering the meanmean of of five ﬁve readings. readings.

TableTable 5. Measurement 5. Measurement accuracy accuracy and uncertainty and uncertainty analysis. analysis.

Sl. No. ParameterSl. No. Parameter Variables Variables Accuracy Accuracy (±) (±) Uncertainty (%) (%) (± (±)) 1 Engine load (N) 0.05 - 1 Accuracy ofEngine the load (N) 0.05 - Accuracy of the2 engine Engine speed (rpm) 2 - 2engine parameter Engine speed (rpm) 2 - parameter3 Fuel flow rate, cc/min 0.1 - 3 Fuel ﬂow rate, cc/min 0.1 - 4 Accuracy of the Hydrocarbon emission - 0.8 4 Hydrocarbon emission - 0.8 Accuracy of5 the obtained emis- Carbon monoxide emission - 1.3 5obtained emission6 sion Carbon value monoxide Oxides emission of nitrogen - - 1.3 1.7 value 67 Determined Oxides pa- ofBrake nitrogen Thermal Efficiency (%) - - 1.7 1 7 8 rametersBrake Thermal HeatEfﬁciency release (%) rate (J/oCA) - - 1.11 Determined parameters 8 Heat release rate (J/oCA) - 1.1 5. Results and Discussions Experimentation was performed on a computerized, single cylinder, naturally aspi- 5. Results and Discussions rated diesel engine by keeping engine speed at 1500 rpm constant. Tests were performed by changingExperimentation the engine was load. performed Viscosity onof athe computerized, biodiesel was single not changed cylinder, too naturally much since aspi- onlyrated 20% diesel of the engine biodiesel by keeping was considered engine speed for atthe 1500 blending; rpm constant. hence, the Tests experiments were performed were performedby changing for theconstant engine injection load. Viscosity pressure of of the 650 biodiesel bar, compression was not changedratio of 18 too and much injection since only 20% of the biodiesel was considered for the blending; hence, the experiments were timing of 23° BTDC. performed for constant injection pressure of 650 bar, compression ratio of 18 and injection ◦ 5.1.timing Brake of Power 23 BTDC. 5.1.The Brake effect Power of graphene nanoparticle and antioxidants at different loads on engine brake Thepower effect for fuel of graphene samples such nanoparticle as diesel, and B20 antioxidantsand B20 with atgraphene different nano loads particle on engine and threebrake antioxidants power for fuel BHT, samples BHA and such TBHQ as diesel, added B20 fuel and samples B20 with is graphene shown in nano Figure particle 2. and three antioxidants BHT, BHA and TBHQ added fuel samples is shown in Figure2.

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Figure 2. Deviation of brakeFigure power 2. withDeviation load. of brake power with load.

It was observed Itthat was even observed though that the even calo thoughrific thevalue calorific and valueviscosity and viscosity of the reformed of the reformed fuel fuel showed poorshowed performance poor performance with reference with reference to the diesel to the and diesel B20, and brake B20, brake power power of the of the engine engine for same reformedfor same reformed fuel showed fuel showed a trivial a trivial surge surge in comparison in comparison with with the the diesel. diesel. The The upturn in upturn in brake powerbrake powerwas because was because of the of secondary the secondary atomization atomization and and complete complete combustion combus- with the inclusion of the graphene nano particle. Additionally, the inclusion of the graphene nano tion with the inclusion of the graphene nano particle. Additionally, the inclusion of the particle restricted the deposition of carbon and iron reducing the friction between the nano graphene nano particleparticle restricted subsequently the increasingdeposition power of carbon [14]. Theand performanceiron reducing of the biodieselfriction in terms between the nanoof particle power and subsequently efficiency is incr alwayseasing lower power than [14]. diesel The fuel performance due to shorter of delay the time and biodiesel in termshigher of power viscosity and ofefficiency the biodiesel is always [21]. Brake lower power than of diesel the engine fuel increasesdue to shorter with an increase delay time and higherin dosage viscosity level of of graphene the biodie nanosel particle,[21]. Brake which power improves of the the engine thermal increases conductivity of the with an increase fuelin dosage and improves level of combustion graphene [13nano] to simaroubaparticle, which biodiesel improves blend. There the thermal was a reduction in conductivity of thebrake fuel power and of improves 3.9% for B20 combustion compared to [13] diesel to and simarouba increase in biodiesel brake power blend. of 5.88%, 6.19% There was a reductionand 6.19% in brake higher power for B20GrBHT1000, of 3.9% for B20 B20GrBHA1000 compared to and diesel B20GrTBHQ1000, and increase respectively,in compared to B20. brake power of 5.88%, 6.19% and 6.19% higher for B20GrBHT1000, B20GrBHA1000 and B20GrTBHQ1000,5.2. respectively, Brake Thermal compared Efficiency to B20. From Figure3, it was observed that the thermal efficiency of B20 was marginally 5.2. Brake Thermalbetter Efficiency than diesel at part load. Additionally, at the full load, the thermal efficiency falls From Figurebelow 3, it was diesel. observed Thermal that efficiency the thermal of graphene efficiency and antioxidants of B20 was showed marginally superior perfor- better than dieselmance at part at load. almost Additionally, all loads in comparable at the full withload, diesel the thermal and B20. efficiency Thermal efficiencyfalls was below diesel. Thermalincreased efficiency by 6.22%, of 3.11%graphene and 3.315%,and antioxidants respectively showed for B20GrBHT1000, superior perfor- B20GrBHA1000 and B20GrTBHQ1000, respectively, higher than B20 at full load. mance at almost all loads in comparable with diesel and B20. Thermal efficiency was increased by 6.22%, 3.11% and 3.315%, respectively for B20GrBHT1000, B20GrBHA1000 and B20GrTBHQ1000, respectively, higher than B20 at full load.

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FigureFigure 3. 3. DeviationDeviation of of brake brake thermal thermal efficiency efﬁciency with brake power power..

5.3. Specific Fuel Consumption 5.3. Specific Fuel Consumption Engines with biodiesel blends consume more fuel for the same power with respect to Engines with biodiesel blends consume more fuel for the same power with respect to diesel and thermal efficiency with the biodiesel blends will be lower than diesel [22] due diesel and thermal efficiency with the biodiesel blends will be lower than diesel [22] due to high density and lower heating value. From Figure 4, it was observed that diesel has to high density and lower heating value. From Figure4, it was observed that diesel has obtainedobtained the the lowest BSFC with 0.33 kg/kW-h at at full full load. load. The The highest highest value value of of BSFC was foundfound for for B20 B20 with with 0.38 0.38 kg/kW-h kg/kW-h with with an an incr increaseease of of15.15% 15.15% compared compared to todiesel. diesel. BSFC BSFC of B20GrBHT1000,of B20GrBHT1000, B20GrBHA1000 B20GrBHA1000 and and B20GrTBHQ1000 B20GrTBHQ1000 were were found found to to be be 0.34, 0.34, 0.36 0.36 and 0.350.35 kg/kW-h, kg/kW-h, respectively. respectively. The The increase increase in in BSFC BSFC for for biodiesel biodiesel blend blend when compared to dieseldiesel was was because because of of the the increase increase in in density density of of the the fuel fuel and and inadequate inadequate atomization atomization of the fuel.fuel. Further, BSFCBSFC ofof the the reformed reformed fuel fuel showed showed lower lower specific specific fuel fuel consumption consumption compared com- paredto B20. to BSFC B20. BSFC of B20GrBHT1000, of B20GrBHT1000, B20GrBHA1000 B20GrBHA1000 and B20GrTBHQ1000 and B20GrTBHQ1000 were were found found to be to11.76%, be 11.76%, 5.55% 5.55% and 8.57% and 8.57% lower lower than thatthanfor that B20. for EvenB20. Even though though the addition the addition of graphene of gra- pheneshowed showed a slight a dipslight in dip the in heating the heating value value of the of fuel, the itfuel, promotes it promotes secondary secondary atomization atomi- zationand accelerates and accelerates combustion combustion by increasing by increasing the power the rendering power rendering slender drop slender in BSFC. drop in BSFC.

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Figure 4. Deviation of brake specific fuel consumption with load. Figure 4. Deviation of brake speciﬁc fuel consumption with load. 5.4. Ignition Delay 5.4. Ignition Delay The main factor while studying the combustion behaviour of any fuel is ignition de- The main factor while studying the combustion behaviour of any fuel is ignition delay. lay. With the increase in engine loading, the ignition delay decreases for all the fuels. A With the increase in engine loading, the ignition delay decreases for all the fuels. A good ci good ci engine fuel should possess lower ignition delay value, which cutbacks the accu- engine fuel should possess lower ignition delay value, which cutbacks the accumulation of mulation of the fuel and sudden burning of the fuel that results in excessive pressure rise the fuel and sudden burning of the fuel that results in excessive pressure rise rendering rendering knocking. Ignition delay is calculated as the difference in terms of crank angle knocking. Ignition delay is calculated as the difference in terms of crank angle between between start of injection and start of combustion. From Figure 5, it was clear that the start of injection and start of combustion. From Figure5, it was clear that the ignition delay ignition delay was lower for biodiesel blend fuels compared to diesel; this was due to the was lower for biodiesel blend fuels compared to diesel; this was due to the higher cetane higherrating ofcetane the biodiesel rating of [ 5the]. Delay biodiesel period [5]. of De thelay biodiesel period of blend the biodiesel tends to be blend lower tends compared to be lowerto pure compared diesel because, to pure of thediesel high because, cetane numberof the high and highercetane oxygennumber content and higher [21,23 oxygen]. There contentwas a decrease [21,23]. inThere the delay was perioda decrease of about in th 5.55%,e delay 11.11% period and of 13.89%about 5.55%, for B20GrBHT1000, 11.11% and 13.89%B20GrBHA1000, for B20GrBHT1000, and B20GrTBHQ1000, B20GrBHA1000, respectively, and B20GrTBHQ1000, compared to B20. respectively, The decrease incom- the pareddelay to period B20. withThe decrease respect to in reformed the delay fuel period was with because respect of the to better reformed atomisation fuel was of because the fuel ofin the the better premixed atomisation phase. of the fuel in the premixed phase.

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FigureFigure 5. 5.DeviationDeviation of of ignition ignition delay withwith respect respect to to loading. loading.

5.5.5.5. Heat Heat Release Release Rate Rate TheThe inclusion inclusion of of graphene graphene nano particleparticle promoted promoted secondary secondary atomisation atomisation of the of fuel the fuel dropletdroplet that that enhanced enhanced the the combustion combustion efﬁciencyefficiency and and improved improved heat heat release release rate. rate. This isThis is becausebecause the the addition addition of of graphe graphenene nanonano particleparticle in in fuel fuel samples samples increases increases higher higher carbon carbon combustioncombustion activation activation and and hence hence aids betterbetter combustion combustion [13 [13].]. Additionally,Additionally, the the increase increase in heatheat transfertransfer was was due due to to the the shorter shorter in ignition in ignition delay delay occursoccurs due due to tothe the micro micro explosion explosion of of the the fuel fuel with thethe inﬂuenceinfluence of of graphene graphene nano nano parti- particle cle [13]. From Figure6, the net heat release rate was improved by 2.39%, 1.26% and 11.66% [13]. From Figure 6, the net heat release rate was improved by 2.39%, 1.26% and 11.66% for B20GrBHT1000, B20GrBHA1000 and B20GrTBHQ1000, respectively, compared to B20. for NetB20GrBHT1000, heat release rate B20GrBHA1000 for B20 dropped and by 3.74%B20GrTBHQ1000, compared to B20. respectively, compared to B20. Net heat release rate for B20 dropped by 3.74% compared to B20. 5.6. Pressure Rise Rate Figures7 and8 show the rate of pressure rises with respect to crank angle and loading, respectively. The rate of pressure rise was highest for B20GrTBHQ1000 for all loading. The pressure rise rate increases with increase in loading. The rate of pressure rise should be less than 9 bar/◦ CA for a stationary engine. It was observed that for the reformed fuel (graphene+antioxidants added fuel) a small increase in the rate of pressure rise was observed. There was an increase in rate of pressure rise of about 4.41% that was observed for B20GrTBHQ1000 compared to diesel. Additionally, the pressure rise rate of about 0.5% and 1.528% was observed for B20GrBHT1000 and B20GrBHA1000, respectively. Higher ignition delay could be the possible reason for the surge in the maximum rate of pressure rise, which causes more entrainment of mixture and results in higher premixed combustion energy [13].

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Energies 2021, 14, 4306 Figure 6. Deviation of heat release rate with crank angle. 12 of 17 Figure 6. Deviation of heat release rate with crank angle.

5.6. Pressure Rise Rate Figures 7 and 8 show the rate of pressure rises with respect to crank angle and loading, respectively. The rate of pressure rise was highest for B20GrTBHQ1000 for all loading. The pressure rise rate increases with increase in loading. The rate of pressure rise should be less than 9 bar/° CA for a stationary engine. It was observed that for the reformed fuel (graphene+antioxidants added fuel) a small increase in the rate of pressure rise was observed. There was an increase in rate of pressure rise of about 4.41% that was observed for B20GrTBHQ1000 compared to diesel. Additionally, the pressure rise rate of about 0.5% and 1.528% was observed for B20GrBHT1000 and B20GrBHA1000, respectively. Higher ignition delay could be the possible reason for the surge in the maximum rate of pressure rise, which causes more entrainment of mixture and results in higher premixed combustion energy [13].

Figure 7. Deviation Figureof pressure 7. Deviation rise rate ofwith pressure crank riseangle. rate with crank angle.

Figure 8. Deviation of pressure rise rate with loading.

5.7. CO Emission Figure 9 shows deviation of carbon monoxide emission with loading for diesel, B20 and graphene plus antioxidant added fuel samples. Carbon monoxide emission increases with an increase in loading. The foremost cause for the same was that the fuel becomes

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Figure 7. Deviation of pressure rise rate with crank angle.

Figure 8. Deviation ofFigure pressure 8. Deviationrise rate with of pressure loading. rise rate with loading.

5.7. CO Emission5.7. CO Emission Figure 9 showsFigure deviation9 shows of carbon deviation monoxi of carbonde emission monoxide with emission loading withfor diesel, loading B20 for diesel, B20 and graphene andplus grapheneantioxidant plus added antioxidant fuel samples. added Carbon fuel samples. monoxide Carbon emission monoxide increases emission increases with an increasewith in anloading. increase The in foremost loading. cause The foremost for the same cause was for thethat same the fuel was becomes that the fuel becomes comparatively richer at higher loads with a richer mixture of carbon content as the fuel increases. From the graph it was learnt that diesel has more CO emission at all loads compared to B20 and reformed fuel. The decline in CO emission for biodiesel blend was owing to higher oxygen concentration in the biodiesel, which ensures complete combustion of the fuel leaving behind the small traces of the CO emission in comparable with the diesel fuel. There was a reduction of 26.68%, 18.53%, 14.46% and 22.6% for B20, B20GrBHT1000, B20GrBHA1000 and B20GrTBHQ1000, respectively, in comparison with the diesel fuel. It was also evident that with the presence of graphene nano particle which increases the chemical reactivity and promotes good combustion by oxidizing CO to CO2 [14]. Correspondingly, the manifestation of antioxidants proved to be a trivial upturn in CO emission. The inclusion of antioxidants increases the CO emission by 14.2%, by lowering the conversion efﬁciency of CO to CO2 [21]. Energies 2021, 14, 4306 13 of 17

comparatively richer at higher loads with a richer mixture of carbon content as the fuel increases. From the graph it was learnt that diesel has more CO emission at all loads compared to B20 and reformed fuel. The decline in CO emission for biodiesel blend was owing to higher oxygen concentration in the biodiesel, which ensures complete combustion of the fuel leaving behind the small traces of the CO emission in comparable with the diesel fuel. There was a reduction of 26.68%, 18.53%, 14.46% and 22.6% for B20, B20GrBHT1000, B20GrBHA1000 and B20GrTBHQ1000, respectively, in comparison with the diesel fuel. It was also evident that with the presence of graphene nano particle which increases the chemical reactivity and promotes good combustion by oxidizing CO to CO2 [14]. Corre- spondingly, the manifestation of antioxidants proved to be a trivial upturn in CO emission. The inclusion of antioxidants increases the CO emission by 14.2%, by lowering the conversion efficiency of CO to CO2 [21]. The formation of hydroxyl radicals because of the oxidation of the peroxyl and hydrogen peroxide decreases the conversion efficiency of CO to CO2 [24,25]. From the present experiments there was an increase of 11.11%, 22.22% and 16.66% of CO for B20GrBHT1000, B20GrBHA1000 and B20GrTBHQ1000, respectively, compared to B20. Energies 2021, 14, 4306The main reason for CO emission upturn can be explained that the addition of BHA, BHT 13 of 17 and TBHQ antioxidants lower the peroxyl and hydrogen peroxide radicals, which have a negative impact on the formation of OH radical as well as oxidation of CO [26].

Figure 9. DeviationFigure of carbon 9. Deviation monoxide of carbon emission monoxide with loading. emission with loading.

5.8. HC Emission The formation of hydroxyl radicals because of the oxidation of the peroxyl and hydro- Hydrocarbongen peroxideemission decreasesfor different the conversionfuel samples efﬁciency is shown of in CO Figure to CO 10.2 [ 24The,25 ].main From the present reason for the experimentshydrocarbon there emission was an is increasethe incomplete of 11.11%, combustion 22.22% and of 16.66%the fuel. of From CO for the B20GrBHT1000, graph, it was evidentB20GrBHA1000 that diesel and has B20GrTBHQ1000, a higher HC emission respectively, at all comparedloads compared to B20. to The B20 main reason for and graphene COplus emission antioxidant upturn added can biodiesel be explained blend. that Owing the addition to the large of BHA, surface BHT area and to TBHQ antioxi- the volume ratiodants of the lower fuel the with peroxyl the presence and hydrogen of nano peroxide particle, radicals,which accelerates which have the a com- negative impact on bustion renderingthe formationample improvement of OH radical in combus as welltion as oxidation of the fuel of and CO minimizing [26]. unburnt hydrocarbon emission. There was a reduction in HC of 42.85%, 64.28% and 71.42% for 5.8. HC Emission B20GrBHT1000, B20GrBHA1000 and B20GrTBHQ1000, respectively, compared to B20. Additionally, the combinedHydrocarbon effect emission of graphe forne differentand antioxidants fuel samples promotes is shown complete in Figure com- 10. The main bustion with minimumreason for hydrocarbon the hydrocarbon emission. emission is the incomplete combustion of the fuel. From the graph, it was evident that diesel has a higher HC emission at all loads compared to B20 and graphene plus antioxidant added biodiesel blend. Owing to the large surface area to the volume ratio of the fuel with the presence of nano particle, which accelerates the

combustion rendering ample improvement in combustion of the fuel and minimizing unburnt hydrocarbon emission. There was a reduction in HC of 42.85%, 64.28% and 71.42% for B20GrBHT1000, B20GrBHA1000 and B20GrTBHQ1000, respectively, compared to B20. Additionally, the combined effect of graphene and antioxidants promotes complete combustion with minimum hydrocarbon emission. Energies 2021, 14, 4306 14 of 17 Energies 2021, 14, 4306 14 of 17

Figure 10. Deviation of hydrocarbon emission with loading. Figure 10. Deviation of hydrocarbon emission with loading.

5.9. NOX Emission 5.9. NOX Emission High combustion temperature is the main culprit for the development of oxides of High combustion temperature is the main culprit for the development of oxides of nitrogen in exhaust emissions. About 95% of the oxides of nitrogen produced in the high nitrogen in exhaust emissions. About 95% of the oxides of nitrogen produced in the high temperature burning process are in the form of nitric oxide [27]. This is because the high temperature burning process are in the form of nitric oxide [27]. This is because the high temperature inside the cylinder splits the triple bond of the nitrogen molecule and makes temperature inside the cylinder splits the triple bond of the nitrogen molecule and makes less reactive atomic nitrogen. This nitrogen again combines with oxygen and forms ther- less reactive atomic nitrogen. This nitrogen again combines with oxygen and forms thermal mal NOx. Biodiesel blends as a fuel increases both oxygen content in the fuel as well as the NO . Biodiesel blends as a fuel increases both oxygen content in the fuel as well as the combustionx temperature. Increased formation of CH radicals during biodiesel combus- combustion temperature. Increased formation of CH radicals during biodiesel combustion tion leads to surge in oxides of nitrogen emission [11]. leads to surge in oxides of nitrogen emission [11]. Biodiesel has a higher amount of oxygen and lower ignition delay than diesel; this Biodiesel has a higher amount of oxygen and lower ignition delay than diesel; this increases the oxides of nitrogen emission compared to diesel.diesel. Inclusion of 1000 ppm of BHT antioxidants antioxidants showed showed a apositive positive impact impact in inreducing reducing the theexhaust exhaust gas gastemperature temperature and renderedand rendered a reduction a reduction in oxides in oxides of nitrogen of nitrogen emission emission to 9.45% to 9.45% [28]. [ 28We]. Welearned learned from from the majoritythe majority of the of previous the previous studies studies that thatthe oxides the oxides of nitrogen of nitrogen emission emission keep keepincreasing increasing even witheven nano with additives nano additives because because of the better of the combustion better combustion and higher and adiabatic higher adiabatic flame temper- ﬂame aturetemperature [29]. From [29 ].the From various the variousstudies [16,21,24,25,28], studies [16,21,24 it,25 was,28 stated], it was that stated the reaction that the between reaction nitrogenbetween molecule nitrogen moleculeand hydrocarbon and hydrocarbon free radicals free are radicals the main are culprit the main in the culprit process in the of formationprocess of formationof oxides of of nitrogen. oxides of The nitrogen. presence The of presence antioxidants of antioxidants minimizes minimizes these reactions these andreactions consequently and consequently lowers oxides lowers of oxidesnitrogen of emission. nitrogen emission. Figure 1111 shows shows variation variation of of oxides oxides of nitrogenof nitrogen emission emission with with respect respect to loading. to loading. From Fromthe ﬁgure the itfigure was clearit was that clear for B20,that oxidesfor B20, of oxid nitrogenes of emissionnitrogen were emission higher were at all higher loads. at With all loads.the increase With the in loading, increase the in combustionloading, the temperaturecombustion becomestemperature higher becomes and results higher in and higher re- sultsppm in of higher oxides ppm of nitrogen. of oxides There of nitrogen. was a reduction There was of a 18.73%, reduction 24.81% of 18.73%, and 27.91% 24.81% lesser and 27.91%oxides oflesser nitrogen oxides emission of nitrogen for B20GrBHT1000, emission for B20GrBHA1000B20GrBHT1000, andB20GrBHA1000 B20GrTBHQ1000, and B20GrTBHQ1000,respectively, compared respectively, to B20. compared Antioxidant to B20. inclusion Antioxidant has shown inclusion a positive has shown impact a pos- on itivereducing impact oxides on reducing of nitrogen oxides emission. of nitrogen emission.

Energies 2021, 14, 4306 15 of 17 Energies 2021, 14, 4306 15 of 17

Figure 11. NOX emission with loading. Figure 11. NOX emission with loading.

6.6. Conclusions Conclusions TheThe motive motive of of the the present present research research is is to to improve improve the the performance performance and and to to reduce reduce emis- emis- sionsion of of biodiesel biodiesel operated operated fuel fuel in in compression compression ignition ignition engine. engine. From From the the extensive extensive survey survey ofof the the previous previous literature, it hashas beenbeen notednoted thatthat the the addition addition of of graphene graphene improves improves perfor- per- formancemance and and emissions, emissions, but but also also there there was was a slight a slight surge surge in thein the oxides oxides of nitrogenof nitrogen emission; emission;the addition the addition of antioxidants of antioxidants lowers lowers the oxidesthe oxides of nitrogen of nitrogen emission. emission. Simultaneously, Simultaneously, CO COemissions emissions surge surge slightly slightly and and performance performanc parameterse parameters are are decreased. decreased. The The outcome outcome of theof thecombined combined effect effect of graphene of graphene and and antioxidants antioxidants studied studied in the in presentthe present study study are in are line in withline withwhat what most most of the of studies the studies reported reported in terms in terms of performance of performance and emission.and emission. BasedBased on on the the experimental experimental test, test, which which was was carried carried out out on on a a single single cylinder cylinder 4 4 stroke stroke compressioncompression ignition ignition engine, engine, for for fuel fuel sample sampless of diesel, of diesel, B20 B20 and andthree three different different monophy- mono- leticphyletic antioxidants antioxidants BHT, BHT, BHA BHA and TBHQ and TBHQ with withgraphene graphene nano nanoparticle, particle, the following the following con- clusionconclusion can be can drawn: be drawn: •• AA 20% 20% blending blending of of biodiesel biodiesel derived derived from from waste waste cooking cooking oil oil met met ASTM ASTM specification specification forfor diesel–biodiesel diesel–biodiesel blend blend and and can can be be used used without without any any modification modification to to the the engine. engine. •• TheThe addition ofof graphene graphene nanoparticle nanoparticle and and BHT, BHT, BHA BHA and TBHQ and TBHQ antioxidants antioxidants yielded yieldeda diminutive a diminutive reduction reduction in calorific in valuecalorific of thevalue fuel of samples the fuel withoutsamples altering without kinematic altering kinematicviscosity andviscosity density and compared density compared to B20. to B20. •• ThereThere was anan increaseincrease in in brake brake thermal thermal efficiency efficiency of 6.22%, of 6.22%, 3.11% 3.11% and 3.315%, and 3.315%, respec- respectively,tively, for B20GrBHT1000, for B20GrBHT1000, B20GrBHA1000 B20GrBHA1000 and B20GrTBHQ1000, and respectively;B20GrTBHQ1000, this is respectively;higher than B20this atis higher full load. than The B20 graphene at full load. nano The particle graphene played nano a particle tremendous played role a tremendoushere for boosting role thehere efficiency, for boosting even the though efficiency, the addition even ofthough antioxidants the addition might notof antioxidantsyield a good might result innot terms yield of a performancegood result in of terms the engine of performance is considered. of the engine is • considered.BSFC of the reformed fuel showed lower specific fuel consumption compared to • BSFCB20. of BSFC the reformed of B20GrBHT1000, fuel showed B20GrBHA1000 lower specific fuel and consumption B20GrTBHQ1000 compared were to found B20. BSFCto be 11.76%,of B20GrBHT1000, 5.55% and 8.57%B20GrBHA1000 lower than and that B20GrTBHQ1000 for B20. Even though were found the addition to be 11.76%,of graphene 5.55% showed and 8.57% a slight lower dip than in the that calorific for B20. value Even of thethough fuel, the it promotes addition theof graphene showed a slight dip in the calorific value of the fuel, it promotes the

Energies 2021, 14, 4306 16 of 17

secondary atomization and accelerates combustion by increasing the power rendering slender drop in BSFC. • There was a reduction in HC of 42.85%, 64.28% and 71.42% and oxides of nitrogen by 18.73%, 24.81% and 27.91% for B20GrBHT1000, B20GrBHA1000 and B20GrTBHQ1000, respectively, compare to B20. • There was a slight increase in CO of about 11.11%, 22.22% and 16.66% for B20GrBHT1000, B20GrBHA1000 and B20GrTBHQ1000, respectively, compared to B20. The inclusion of antioxidants lowers the peroxyl and hydrogen peroxide radicals, which has a negative impact on the formation of OH radical as well as oxidation of CO. • There was a reduction of 18.73%, 24.81% and 27.91% lesser oxides of nitrogen emission for B20GrBHT1000, B20GrBHA1000 and B20GrTBHQ1000, respectively, compared to B20. Antioxidant inclusion showed a positive impact on reducing oxides of nitrogen emission. • B20GrTBHQ1000 has yielded the best result in terms of power, efﬁciency and emission.

Author Contributions: Conceptualization, S.K. and C.R.R.; methodology, S.K. and T.M.Y.K.; valida- tion, M.E.M.S., A.A.; formal analysis, T.M.Y.K. and C.R.R.; investigation, S.K.; resources, C.R.R. and A.E.; data curation, A.A.; writing—original draft preparation, S.K. and C.R.R.; writing—review and editing, C.R.R. and T.M.Y.K.; visualization, M.E.M.S.; supervision, C.R.R.; project administration, T.M.Y.K.; funding acquisition, T.M.Y.K. and A.E. All authors have read and agreed to the published version of the manuscript. Funding: Deanship of Scientific Research at King Khalid University, Research grant number “R.G.P 2/107/41”. This work was also supported by Taif University researchers supporting project number (TURSP–2020/40), Taif University, Taif, Saudi Arabia. Acknowledgments: The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through research groups program under grant number R.G.P 2/107/41. This work was also supported by Taif University researchers supporting project number (TURSP–2020/40), Taif University, Taif, Saudi Arabia. Conflicts of Interest: The authors declare no conflict of interest.

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